HIV is the virus that is responsible for the worldwide AIDS epidemic. The initial stages of HIV infection involve the fusion of the viral membrane with the target cell membrane, a process that injects the viral contents into the cellular cytoplasm. On the viral side, the molecular complex responsible for the fusion activity contains the surface protein gp120 and the transmembrane protein gp41. It is currently believed that gp120 interacts with the proteins CD4 and coreceptors on the target cell, resulting in a conformational change that causes gp41 to insert its amino terminus (fusion peptide region) into the target cell membrane. This structural rearrangement promotes the fusion of virus and cellular membranes through a poorly understood mechanism.